Aerial imaging overlay technology has been a reassuringly familiar feature of semiconductor manufacturing for years, experiencing little change. The wafers are loaded, special box-in-box (BiB) structures or targets are examined using a white-light microscope, and the images are then processed to determine overlay error. Although this has worked well, there are several issues as the industry moves toward implementing deep nanometer process levels with for example image overlay tools, location of alignment marks used in a stage metrology system of lithography tool or inspection tool, and measurement of properties such as CDs.
With respect to overlay metrology, one of these issues has to do with the traditionally used BiB structures which tend to have large geometries and typically consist of bars microns in size that do not correspond to circuit feature sizes. Additionally, the BiB structures tend to have large areas of low feature density which polish at different rates than circuit areas with high feature density. As increasingly smaller features are being produced, it has become apparent that the behavior of these traditional BiB targets does not accurately reflect that of the circuit features themselves.
This imaging overlay uncertainty has been tolerable because process windows have not had to be too tight and the available precision and accuracy have been adequate. Now, smaller geometries are resulting in accuracy issues that must be addressed in imaging overlay tools, in particular the tool-induced shift (TIS). For imaging overlay tools, TIS can be on the order of several nanometers. At the 65 nm node, 2 or 3 nm of TIS is a significant measurement uncertainty.
Imaging overlay makes it difficult to reduce TIS because it is critically dependent upon residual aberrations in the imaging overlay optics with the most critical one being the microscope objective. Strehl ratios as high as 0.9 often do not provide the TIS performance required for advanced design rules. Thus, the technology faces not only optical problems but the fact that traditional overlay marks do not represent the actual behavior of circuit features that are orders of magnitude smaller.
Overlay metrology is the art of checking the quality of alignment after lithography. Overlay error is defined as the offset between two patterned layers from their ideal relative position. An overlay error is a vector quantity with two components in the plane of the wafer. Depending on the context, overlay error may signify one of the components or the magnitude of the vector.
Overlay metrology saves subsequent process steps that would be built on a faulty foundation in case of an alignment error. Overlay metrology provides the information that is necessary to correct the alignment of the stepper-scanner and thereby minimize overlay error on subsequent wafers. Moreover, overlay errors detected on a given wafer after exposing and developing the photoresist can be corrected by removing the photoresist and repeating the lithography step on a corrected stepper-scanner. If the measured error is minor, parameters for subsequent steps of the lithography process could be adjusted based on the output of the overlay metrology to avoid excursions. If overlay error is measured subsequently, e.g. after the etch step that typically follows the develop step, it can be used to “scrap” severely mis-processed wafers, or to adjust process equipment for better performance on subsequent wafers, i.e. APC.
As the CD in IC device reduces, the total overlay budget needs to be more stringent. Typically, the allowable overlay error is ⅓ of the CD in the IC device. In this case, robustness of alignment mark is critical as accurate signal is required by the scanner's alignment system to precisely align a pattern of a layer to the pattern of the previous layer. Alignment issue is more severe in a back-end process partly due to the influence of CMP, which contributes to the asymmetric or total destruction of the alignment marks. The corresponding error in overlay accuracy has been called wafer induced shift (WIS).
Alignment marks and patterns used in overlay metrology can be placed on/in the scribe-line between die sites on a wafer in order not to waste the usable area on the wafer. Dimensions of a standard scribe-line are 80 microns. Preferably, the alignment marks and the patterns should be as small as possible so as to create the option of reducing the size of scribe-lines and subsequently lead to an increase in the usable area on the wafer.
However, reduced physical size of alignment marks and of overlay patterns used in overlay metrology reduces the magnitude of the diffracted signals detected in determination of alignment marks and overlay errors, respectively. Thus it is desirable to have an alignment mark metrology system and an overlay metrology system that meet the overlay accuracy requirements, have reduced TIS, can compensate for WIS, and reduces the area required for the alignment marks and the overlay patterns.
The problems that arise in an optical measurement of a CD can also include those introduced by TIS and WIS. Thus an overlay metrology system and an alignment mark metrology system that successfully address the affects of TIS and WIS for a given technology node can be adapted for use in an improved CD metrology system.
Defects of different types can affect the performance of overlay, alignment mark, and CD metrology systems. Accordingly, it is desirable for the respective metrology systems to be able to detect the presence of defects and take appropriate measures when the presence of a defect is detected.
Prior overlay metrology methods use built-in test patterns etched or otherwise formed into or on the various layers during the same set of lithography steps that form the patterns for circuit elements on the wafer. The typical BiB pattern consists of two concentric squares, formed on a lower and an upper layer, respectively. “Bar-in-bar” is a similar pattern with just the edges of the “boxes” demarcated, and broken into disjoint line segments. Typically one is the upper pattern and the other is the lower pattern, i.e. corresponding to earlier and later steps in the process. There are other patterns used for overlay metrology. The squares or bars are formed by lithographic and other processes used to make planar structures, e.g., CMP. Currently, the patterns for the boxes or bars are stored on lithography masks and projected onto the wafer. Other methods for putting the patterns on the wafer are possible, e.g. direct electron beam writing from computer memory, etc.
In one form of the prior art, a high performance microscope imaging system combined with image processing software estimates overlay error for the two layers. The image processing software uses the intensity of light at a multitude of pixels. Obtaining the overlay error accurately requires a high quality imaging system and means of focusing it. Some of this prior art is reviewed by the article “Semiconductor Pattern Overlay”, by Neal T. Sullivan, Handbook of Critical Dimension Metrology and Process Control, Kevin M. Monahan, ed., SPIE Optical Engineering Press, CR52, pp 160. A. Starikov, D. J. Coleman, P. J. Larson, A. D. Lapata, and W. A. Muth in “Accuracy of Overlay Measurements: Tool and Mark Asymmetry Effects,” Optical Engineering, 31, pp 1298 (1992), teach measuring overlay at one wafer orientation, rotating the wafer by 180°, measuring overlay again and attributing the difference to tool errors and overlay mark asymmetry.
One requirement for the optical system is very stable positioning of the optical system with respect to the sample. Relative vibration would blur the image and degrade the performance. This is a difficult requirement to meet for overlay metrology systems that are integrated into a process tool. The tool causes potentially large accelerations (vibrations), e.g. due to high acceleration wafer handlers. The tight space requirements for integration preclude bulky isolation strategies.
The imaging based overlay measurement precision can be two orders of magnitude smaller than the wavelength of the light used to image the target patterns of concentric boxes or bars. At such small length scales, the image does not have well determined edges because of diffraction. The determination of the edge, and therefore the overlay measurement, is affected by any factor that changes the diffraction pattern. CMP is a commonly used technique to planarize the wafer surface at intermediate process steps before depositing more material. CMP can render the profile of the trenches or lines that make up the overlay measurement targets asymmetric. The asymmetry in target changes the diffraction pattern, thus potentially causing an overlay measurement error.
In U.S. Pat. No. 4,757,207, Chappelow, et al. teach obtaining the quantitative value of the overlay offset from the reflectance of targets that consists of identical line gratings that are overlaid upon each other on a planar substrate. Chappelow et al. approximate the reflectance of the overlapping gratings as the average of the reflectances of four film stacks weighted by their area-fractions. This approximation, which neglects diffraction, has some validity when the lines and spaces are larger than largest wavelength of the reflectometer. The reflectance of each of the four film stacks is measured at a so called macro-site close to the overlay target. Each macro-site has a uniform film stack over a region that is larger than the measurement spot of the reflectometer. A limitation of U.S. Pat. No. 4,757,207 is that spatial variations in the film thickness that are caused by CMP and resist loss during lithography will cause erroneous overlay measurements.
Another limitation of U.S. Pat. No. 4,757,207 is that reflectance is measured at eight sites in one overlay metrology target, which increases the size of the target and decreases the throughput of the measurement. Another limitation of U.S. Pat. No. 4,757,207 is that the lines and spaces need to be large compared to the wavelength, but small compared to the measurement spot which limits the accuracy and precision of the measurement.
Another limitation of U.S. Pat. No. 4,757,207 is that the light intensity is measured by a single photodiode. The dependence of the optical properties of the sample is not measured as a function of wavelength, or angle of incidence, or polarization, which limits the precision of the measurement.
The “average reflectivity” approximation for the interaction of light with gratings, as employed by U.S. Pat. No. 4,757,207, greatly simplifies the problem of light interaction with a grating but neglects much of the diffraction physics. The model used to interpret the data has four distinct regions whose respective reflectivities are determined by the combination of layers formed by the substrate and the overlaid patterns and by the respective materials in the substrate and patterns. Eq. (1) in the U.S. Pat. No. 4,757,207 clearly indicates that these regions do not interact, i.e. via diffraction, as the total reflectivity of the structure is a simple average of the four reflectivities with area weighting.
IBM Technical Disclosure Bulletin 90A 60854/GE8880210, March 1990, pp 170, teaches measuring offset between two patterned layers by overlapping gratings. There are four sets of overlapping gratings to measure the x-offset and another four sets of overlapping gratings to measure the y-offset. The four sets of gratings, which are measured by a spectroscopic reflectometer, have offset biases of 0, ¼, ½, ¾-pitch. GE880210 does not use a model that accounts for the diffraction of light by the gratings or the multiple scattering of the light by the two gratings, and it has no provision to handle non-rectangular line profiles.
In U.S. Pat. No. 6,150,231, Muller et al. teach measuring overlay by Moire patterns. The Moire pattern is formed by overlapping gratings patterns, one grating on the lower level, another on the upper level. The two grating patterns have different pitches. The Moire pattern approach requires imaging the overlapping gratings and estimating their offset from the spatial characteristics of the image.
In U.S. Pat. Nos. 6,023,338 and 6,079,256, Bareket teaches an alternative approach in which two complementary periodic grating structures are produced on the two subsequent layers that require alignment. The two periodic structures are arranged adjacent to and in fixed positions relative to one another, such that there is no overlap of the two structures. The two gratings are scanned, either optically or with a stylus, so as to detect the individual undulations of the gratings as a function of position. The overlay error is obtained from the spatial phase shift between the undulations of the two gratings.
Smith et al. in U.S. Pat. No. 4,200,395 and Ono in U.S. Pat. No. 4,332,473 teach aligning a wafer and a mask by using overlapping diffraction gratings and measuring higher order, i.e. non-specular, diffracted light. One diffraction grating is on the wafer and another one is on the mask. The overlapping gratings are illuminated by a normally incident light and the intensities of the positive and negative diffracted orders, e. g. 1st and −1st orders, are compared. The difference between the intensities of the 1st and −1st diffracted orders provides a feedback signal which can be used to align the wafer and the mask. These inventions are similar to the International (PCT) Application Publication No. WO 02/065545 A2 in that they use overlapping gratings on two layers. However, the U.S. Pat. No. 4,200,395 and U.S. Pat. No. 4,332,473 patents are applicable to mask alignment but not to overlay metrology. They do not teach how to obtain the quantitative value of the offset from the light intensity measurements. U.S. Pat. Nos. 4,200,395 and 4,332,473 are not applicable to a measurement system that only uses specular, i.e. zeroth-order diffracted light.
WO 02/065545 A2 teaches measuring overlay by scatterometry. Measurements of structural parameters of a diffracting structure from optical characterization are now well known in the art as scatterometry. With such methods, a measurement sample is illuminated with optical radiation, and the sample properties are determined by measuring characteristics of the scattered radiation (e.g. intensity, polarization state, or angular distribution). A diffracting structure consists of one or more layers that may have lateral structure within the illuminated and detected area, resulting in diffraction of the reflected (or transmitted) radiation. If the lateral structure dimensions are smaller than the illuminating wavelengths, then diffracted orders other than the zeroth order may all be evanescent and not directly observable. But the structure geometry can nevertheless significantly affect the zeroth-order reflection, making it possible to make optical measurements of structural features much smaller than the illuminating wavelengths.
In one type of measurement process of WO 02/065545 A2, a microstructure is illuminated and the intensity of reflected or diffracted radiation is detected as a function of the radiation's wavelength, the incidence direction, the collection direction, or polarization state (or a combination of such factors). Direction is typically specified as a polar angle and azimuth, where the reference for the polar angle is the normal to the wafer and the reference for the azimuth is either some pattern(s) on the wafer or other marker, e.g. a notch or a flat for silicon wafers. The measured intensity data is then passed to a data processing machine that uses some model of the scattering from possible structures on the wafer. For example, the model may employ Maxwell's equations to calculate the theoretical optical characteristics as a function of measurement parameters (e.g. film thickness, line width, etc.), and the parameters are adjusted until the measured and theoretical intensities agree within specified convergence criteria. The initial parameter estimates may be provided in terms of an initial “seed” model of the measured structure. Alternatively, the optical model may exist as pre-computed theoretical characteristics as a function of one or more measurement parameters in tabular form, i.e. a “library”, that associates collections of parameters with theoretical optical characteristics. The “extracted” structural model has the structural parameters associated with the optical model which best fits the measured characteristics, e.g. in a least-squares sense.
Conrad (U.S. Pat. No. 5,963,329) uses scatterometry to measure the line profile or topographical cross-sections. The direct application of Maxwell's equations to diffracting structures, in contrast to non-diffracting structures (e.g. unpatterned films), is much more complex and time-consuming, possibly resulting in either a considerable time delay between data acquisition and result reporting and/or the need to use a physical model of the profile which is very simple and possibly neglects significant features.
Scheiner et al. (U.S. Pat. No. 6,100,985) teaches a measurement method that is similar to that of Conrad, except that Scheiner's method uses a simplified, approximate optical model of the diffracting structure that does not involve direct numerical solution of Maxwell's equations. This avoids the complexity and calculation time of the direct numerical solution. However, the approximations inherent in the simplified model make it inadequate for grating structures that have period and line width dimensions comparable to or smaller than the illumination wavelengths.
In an alternative method taught by McNeil et al. (U.S. Pat. No. 5,867,276) the calculation time delay is substantially reduced by storing a multivariate statistical analysis model based on calibration data from a range of model structures. The calibration data may come from the application of Maxwell's equations to parameterized models of the structure. The statistical analysis is applied to the measured diffraction characteristics and returns estimates of the parameters for the actual structure.
The measurement method taught by McNeil et al. uses diffraction characteristics consisting of spectroscopic intensity data. A similar method can also be used with ellipsometric data, using ellipsometric parameters such as tan ψ and cos Δ, in lieu of intensity data. For example, X. Niu in “Specular Spectroscopic Scatterometry in DUV Lithography,” Proc. SPIE 3677, pp. 159 (1999), uses a library approach. The library method can be used to simultaneously measure multiple model parameters (e.g. linewidth, edge slope, film thickness).
In International (PCT) Application Publication No. WO 99/45340, Xu et al. disclose a method for measuring the parameters of a diffracting structure on top of laterally homogeneous, non-diffracting films. The disclosed method first constructs a “reference database” based on a priori information about the refractive index and film thickness of underlying films, e.g. from spectroscopic ellipsometry or reflectometry. The reference database has “diffracted light fingerprints” or “signatures” (either diffraction intensities, or alternatively ellipsometric parameters) corresponding to various combinations of grating shape parameters. The grating shape parameters associated with the signature in the reference database that matches the measured signature of the structure are then reported as the grating shape parameters of the structure.
In International (PCT) Application Publication No. WO 02/065545 A2 by A. Sezginer, K. Johnson, and F. E. Stanke and entitled “Overlay Alignment Metrology Using Diffraction Gratings,” alignment accuracy between two patterned layers is measured using a metrology target comprising substantially overlapping diffraction gratings formed in a test area of the layers being tested. An optical instrument illuminates all or part of the target area and measures the optical response. The instrument can measure transmission, reflectance, and/or polarization of the illumination and detected light. Overlay error or offset between those layers containing the test gratings is determined by a processor programmed to calculate an optical response for a set of parameters that include overlay error, using a model that accounts for diffraction by the gratings and interaction to the gratings with each others' diffracted field. The model parameters might also take account of manufactured asymmetries. The calculated and measured responses are iteratively compared and the model parameters changed to minimize the difference.
In International (PCT) Application Publication No. WO 02/069390 by X. Niu and N. Jakatdar and entitled “Grating Test Patterns And Methods For Overlay Metrology,” a metrology is described for determining bias or overlay error in lithographic processes. The metrology includes a set of diffraction test patterns, optical inspection techniques using spectroscopic ellipsometer or reflectometer, and a method of test pattern profile extraction. The metrology uses a set of diffraction gratings as the test patterns and thin film metrology equipment, such as spectroscopic ellipsometer or spectroscopic reflectometer. The profiles of the test patterns in the two successive layers are analyzed. Overlay information are obtained after processing the profile data. In procedure, a line-on-line overlay grating test patterns structure is described in which a second layer mask is placed in the center of a clear line in a first layer mask. In a second procedure, a line-on-line overlay grating test patterns structure is described in which a second layer mask is placed in the center of a dark line in the first mask.
In International (PCT) Application Publication No. WO 02/24723 A2 by B. Brill, M. Finarov, and D. Scheiner and entitled “Lateral Shift Measurement Using An Optical Technique,” a method is described for controlling alignment of layers in a multi-layer sample, such as in semiconductor wafers based on detecting a diffraction efficiency of radiation diffracted from the patterned structures located one above the other in two different layers of the sample.
OCDR is used to measure surface profiles of wafers such as described in the article “Optical Coherency-Domain Reflectometry: A New Optical Evaluation Technique” by R. C. Youngquist, S. Carr, and D. E. N. Davies, Opt. Lett., 12. pp. 158 (1987). OCDR of prior art yields accurate information about the height profile of a surface but does not yield corresponding accurate information about the transverse location of features on a patterned wafer.